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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Integer quantum Hall effect and enhanced g factor in quantum-confined Cd 3 As 2 films

Here, we investigate the integer quantum Hall effect in Cd 3 ⁢As 2 thin films under conditions of strong to moderate quantum confinement (thicknesses of 10, 12, and 15 nm). In all the films, we observe the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor v = 1) and at spin-degenerate higher index Landau levels with even filling factors (v = 2,4,6). With increasing quantum confinement, we also observe a lifting of the Landau-level spin degeneracy at v = 3, manifest as the emergence of an anomaly in the longitudinal and Hall resistivities. Tight-binding calculations show that the enhanced g factor likely arises from a combination of quantum confinement and corrections from nearby subbands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Wet chemical synthesis of quantum confined nanostructured tin oxide thin films by successive ionic layer adsorption and reaction technique

Graphical abstract: - Highlights: • Quantum confined SnO{sub 2} thin films were synthesized at 80 °C by SILAR technique. • Film formation mechanism is discussed. • Films with snow like crystallite morphology offer high specific surface area. • The blue-shifted value of band gap confirmed the quantum confinement effect. • Present synthesis has advantages – low cost, low temperature and green friendly. - Abstract: Quantum confined nanostructured SnO{sub 2} thin films were synthesized at 353 K using ammonium chloride (NH{sub 4}Cl) and other chemicals by successive ionic layer adsorption and reaction technique. Film formation mechanism is discussed. Structural, morphological, optical and electrical properties were investigated and compared with the as-grown and annealed films fabricated without NH{sub 4}Cl solution. SnO{sub 2} films were polycrystalline with crystallites of tetragonal structure with grain sizes lie in the 5–8 nm range. Films with snow like crystallite morphology offer high specific surface area. The blue-shifted value of band gap of as-grown films confirmed the quantum confinement effect of grains. Refractive index of the films lies in the 2.1–2.3 range. Films prepared with NH{sub 4}Cl exhibit relatively lower resistivity of the order of 10{sup 0}–10{sup −1} Ω cm. The present synthesis has advantages such as low cost, low temperature and green friendly, which yields small particle size, large surface–volume ratio, and high crystallinity SnO{sub 2} films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Quantum-Confined Lifshitz Transition on Weyl Semimetal T d -MoTe 2

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal T d -MoTe 2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

36 MATERIALS SCIENCE↗
NASA NTRS2000

Efficient Multi-Dimensional Simulation of Quantum Confinement Effects in Advanced MOS Devices

We investigate the density-gradient (DG) transport model for efficient multi-dimensional simulation of quantum confinement effects in advanced MOS devices. The formulation of the DG model is described as a quantum correction to the classical drift-diffusion model. Quantum confinement effects are shown to be significant in sub-100nm MOSFETs. In thin-oxide MOS capacitors, quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion or quantum effects in simulations dramatically improves the match between C-V simulations and measurements for oxide thickness down to 2 nm. Significant quantum corrections also occur in the I-V characteristics of short-channel (30 to 100 nm) n-MOSFETs, with current drive reduced by up to 70%. This effect is shown to result from reduced inversion charge due to quantum confinement of electrons in the channel. Also, subthreshold slope is degraded by 15 to 20 mV/decade with the inclusion of quantum effects via the density-gradient model, and short channel effects (in particular, drain-induced barrier lowering) are noticeably increased.

Biegel, Bryan A.↗
NASA NTRS2000

Efficient Multi-Dimensional Simulation of Quantum Confinement Effects in Advanced MOS Devices

We investigate the density-gradient (DG) transport model for efficient multi-dimensional simulation of quantum confinement effects in advanced MOS devices. The formulation of the DG model is described as a quantum correction ot the classical drift-diffusion model. Quantum confinement effects are shown to be significant in sub-100nm MOSFETs. In thin-oxide MOS capacitors, quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion of quantum effects may reduce gate capacitance by 25% or more. As a result, the inclusion of quantum effects in simulations dramatically improves the match between C-V simulations and measurements for oxide thickness down to 2 nm. Significant quantum corrections also occur in the I-V characteristics of short-channel (30 to 100 nm) n-MOSFETs, with current drive reduced by up to 70%. This effect is shown to result from reduced inversion charge due to quantum confinement of electrons in the channel. Also, subthreshold slope is degraded by 15 to 20 mV/decade with the inclusion of quantum effects via the density-gradient model, and short channel effects (in particular, drain-induced barrier lowering) are noticeably increased.

Biegel, Bryan A.↗
DOE OSTI2019

Quantum Confined Stark Effect and Temperature Dependencies of Photoluminescence of InAs Quantum Dots Coupled with AlGaAs/GaAs Two Dimensional Electron Gas

In this work, Experimental study of the influence of internal electric field and the temperature on the photoluminescence of InAs self assembled quantum dots inserted in AlGaAs/GaAs modulation doped hetero-structure have been investigated. The internal electric field is controlled by an appropriate design of the hetero-structure. We have observed a red shift of the photoluminescence position peaks result from the quantum confined Stark effect due to the local electric field existing in the structure. Estimation values of the internal electric field have been obtained through carrier’s densities values in interface of AlGaAs/GaAs hetero-junction. An anomalous dependence of the full width at half maximum with temperature has been found, which attributed to the carrier’s dynamics between InAs quantum dots layer and the two-dimensional electron gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Self-screening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers

InGaN/GaN light-emitting diodes (LEDs) grown along the polar orientations significantly suffer from the quantum confined Stark effect (QCSE) caused by the strong polarization induced electric field in the quantum wells, which is a fundamental problem intrinsic to the III-nitrides. Here, we show that the QCSE is self-screened by the polarization induced bulk charges enabled by designing quantum barriers. The InN composition of the InGaN quantum barrier graded along the growth orientation opportunely generates the polarization induced bulk charges in the quantum barrier, which well compensate the polarization induced interface charges, thus avoiding the electric field in the quantum wells. Consequently, the optical output power and the external quantum efficiency are substantially improved for the LEDs. The ability to self-screen the QCSE using polarization induced bulk charges opens up new possibilities for device engineering of III-nitrides not only in LEDs but also in other optoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Suppression of the quantum-confined Stark effect in Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N corrugated quantum wells

We report comparative studies of 6-nm-thick Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N pyroelectric quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on c-sapphire substrates with a thick AlN buffer deposited under different growth conditions. The Al-rich growth conditions result in a 2D growth mode and formation of a planar QW, whereas the N-rich conditions lead to a 3D growth mode and formation of a QW corrugated on the size scale of 200–300 nm. Time-resolved photoluminescence (PL) measurements reveal a strong quantum-confined Stark effect in the planar QW, manifested by a long PL lifetime and a red shift of the PL line. In the corrugated QW, the emission line emerges 200 meV higher in energy, the low-temperature PL lifetime is 40 times shorter, and the PL intensity is stronger (∼4 times at 4.5 K and ∼60 times at 300 K). The improved emission properties are explained by suppression of the quantum-confined Stark effect due to the reduction of the built-in electric field within the QW planes, which are not normal to the [0001] direction, enhanced carrier localization, and improved efficiency of light extraction.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Quantum confinement effects across two-dimensional planes in MoS{sub 2} quantum dots

The low quantum yield (∼10{sup −5}) has restricted practical use of photoluminescence (PL) from MoS{sub 2} composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS{sub 2} quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS{sub 2} QDs is also found to be sensitive to aggregation due to the size effect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Influence of quantum-confined Stark effect on optical properties within trench defects in InGaN quantum wells with different indium content

The trench defects in InGaN/GaN multiple quantum well structures are studied using confocal photoluminescence (PL) spectroscopy and atomic force microscopy. A strong blueshift (up to ∼280 meV) and an intensity increase (by up to a factor of 700) of the emission are demonstrated for regions enclosed by trench loops. The influence of the difference in the well width inside and outside the trench loops observed by transmission electron microscopy, the compositional pulling effect, the strain relaxation inside the loop, and corresponding reduction in the built-in field on the PL band peak position and intensity were estimated. The competition of these effects is mainly governed by the width of the quantum wells in the structure. It is shown that the PL band blueshift observed within the trench defect loops in the InGaN structures with wide quantum wells is mainly caused by the reduction in efficiency of the quantum-confined Stark effect due to strain relaxation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Quantum confinement effect in cheese like silicon nano structure fabricated by metal induced etching

Quantum confinement effect has been studied in cheese like silicon nano-structures (Ch-SiNS) fabricated by metal induced chemical etching using different etching times. Scanning electron microscopy is used for the morphological study of these Ch-SiNS. A visible photoluminescence (PL) emission is observed from the samples under UV excitation at room temperature due to quantum confinement effect. The average size of Silicon Nanostructures (SiNS) present in the samples has been estimated by bond polarizability model using Raman Spectroscopy from the red-shift observed from SiNSs as compared to its bulk counterpart. The sizes of SiNS present in the samples decreases as etching time increase from 45 to 75 mintunes.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Synthesis of Quantum-Confined Borophene Nanoribbons

Here, borophene nanoribbons (BNRs) are one-dimensional strips of atomically thin boron expected to exhibit quantum-confined electronic properties that are not present in extended two-dimensional borophene. While the parent material borophene has been experimentally shown to possess anisotropic metallicity and diverse polymorphic structures, the atomically precise synthesis of nanometer-wide BNRs has not yet been achieved. Here, we demonstrate the synthesis of multiple BNR polymorphs with well-defined edge configurations within the nanometer-scale terraces of vicinal Ag(977). Through atomic-scale imaging, spectroscopy, and first-principles calculations, the synthesized BNR polymorphs are characterized and found to possess distinct edge structures and electronic properties. For single-phase BNRs, v 1/6 -BNRs and v 1/5 -BNRs adopt reconstructed armchair edges and sawtooth edges, respectively. In addition, the electronic properties of single-phase v 1/6 -BNRs and v 1/5 -BNRs are dominated by Friedel oscillations and striped moiré patterns, respectively. On the other hand, mixed-phase BNRs possess quantum-confined states with increasing nodes in the electronic density of states at elevated biases. Overall, the high degree of polymorphism and diverse edge topologies in borophene nanoribbons provide a rich quantum platform for studying one-dimensional electronic states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Twinned silicon and germanium nanocrystals: Formation, stability and quantum confinement

Although twins are often observed in Si/Ge nanocrystals (NCs), little theoretical investigation has been carried out to understand this type of important planar defects in Si/Ge NCs. We now study the twinning of Si/Ge NCs in the frame work of density functional theory by representatively considering single-twinned and fivefold-twinned Si/Ge NCs. It is found that the formation of twinned Si/Ge NCs is thermodynamically possible. The effect of twinning on the formation of Si NCs is different from that of Ge NCs. For both Si and Ge NCs twinning enhances their stability. The quantum confinement effect is weakened by twinning for Si NCs. Twinning actually enhances the quantum confinement of Ge NCs when they are small (<136 atoms), while weakening the quantum confinement of Ge NCs as their size is large (>136 atoms). The current results help to better understand the experimental work on twinned Si/Ge NCs and guide the tuning of Si/Ge-NC structures for desired properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Influence of quantum confinement and strain on orbital polarization of four-layer LaNiO 3 superlattices: A DFT+DMFT study

Atomically precise superlattices involving transition metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substate) and quantum confinement (controlled by layer thickness). We use the combination of density functional theory and dynamical mean field theory (DFT+DMFT) to study Ni E g d-orbital polarization in strained LaNiO 3 /LaAlO 3 superlattices consisting of four layers of nominally metallic NiO 2 and four layers of insulating AlO 2 separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. We determined that the effect of strain is from the dependence of the results on the Ni-O bondlength ratio and the octahedral rotation angles; quantum confinement is studied by comparison to bulk calculations with similar degrees of strain; correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent X-ray absorption spectroscopy and resonant reflectometry data. But, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO 3 and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Strong carrier localization and diminished quantum-confined Stark effect in ultra-thin high-indium-content InGaN quantum wells with violet light emission

Here, we report on the optical and structural characteristics of violet-light-emitting, ultra-thin, high-Indium-content (UTHI) InGaN/GaN multiple quantum wells (MQWs), and of conventional low-In-content MQWs, which both emit at similar emission energies though having different well thicknesses and In compositions. The spatial inhomogeneity of In content, and the potential fluctuation in high-efficiency UTHI MQWs were compared to those in the conventional low-In-content MQWs. We conclude that the UTHI InGaN MQWs are a promising structure for achieving better quantum efficiency in the visible and near-ultraviolet spectral range, owing to their strong carrier localization and reduced quantum-confined Stark effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Tunable Light-Emitting Diodes Utilizing Quantum-Confined Layered Perovskite Emitters

Organic–inorganic perovskites have been shown to have excellent optoelectronic properties. Further, layered perovskites have been demonstrated, utilizing quantum confinement to achieve emission blue-shifted from the bulk band gap. Here, we tune this blue-shift to build LEDs that span the visible spectrum. We demonstrate that electroluminescence from red-shifted layers dominates emission from mixed-thickness devices and that the addition of excess ligand is necessary to drive emission toward blue-shifted layers. By tuning the thickness of the layers, we build LEDs with blue emission utilizing the lead bromide system and orange emission utilizing the lead iodide system. Lastly, we demonstrate that these materials suffer reversible degradation under an applied electric field. The spectrally narrow emission, combined with the favorable electronic properties of perovskite materials and access to shorter emission wavelengths through quantum confinement, demonstrates the promise of these materials as a new platform for low-cost, high-performance light-emitting devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Controlling Carrier Dynamics using Quantum-Confined Semiconductor Nanocrystals

The articles included in this special issue of Chemical Physics explore the use of quantum-confined semiconductor nanocrystals to control the flow of energy and/or charge. Colloidal quantum-confined semiconductor nanostructures are an emerging class of functional materials being developed for novel opto-electronic applications. In the last few years numerous examples in the literature have emerged where novel nanostructures have been tailored such as to achieve a specific function thus moving the field from the stage of discovery of novel behaviors to that of control of nanostructure properties. In addition to the internal structure of the NCs their assemblies can be tailored to achieve emergent properties and add additional control parameters that determine the final opto-electronic properties. These principles are explored via variations in shape, size, surface ligands, heterostructuring, morphology, composition, and assemblies and are demonstrated through measurements of excited state processes, such as Auger recombination; photoluminescence; charge separation and charge transport.

14 SOLAR ENERGY↗